Method of writing a four-transistor memory cell array

ABSTRACT

A pair of cross-coupled inverters that hold a digital state are powered by supplies that also function as row select and column bit lines. A method of reading and writing the digital state of an individual cell, row of cells, or column of cells by manipulating these row-supply or column-supply lines. Reads and writes may be performed on either a row or column basis. A method for reading and logically OR&#39;ing or AND&#39;ing an entire row or column of cells. A method for querying on a row or column basis to function as a content addressable memory (CAM).

CROSS-REFERENCE TO RELATED APPLICATIONS

A number of related copending United States patent applications commonlyowned by the assignee of the present document and incorporated byreference in their entirety into this document is being filed in theUnited States Patent and Trademark Office on or about the same day asthe present application. These related applications are: Ser. No.10/061,917, titled “A FOUR-TRANSISTOR STATIC MEMORY CELL ARRAY”; Ser.No. 10/062,074, titled “A METHOD OF READING A FOUR-TRANSISTOR MEMORYCELL ARRAY”; Ser. No. 10/061,876, titled “A METHOD OF READING ANDLOGICALLY OR'ING OR ANDING A FOUR-TRANSISTOR MEMORY CELL ARRAY BY ROWSOR COLUMNS”; and, Ser. No. 10/062,053, titled “A METHOD OF QUERYING AFOUR-TRANSISTOR MEMORY ARRAY LIKE A CONTENT-ADDRESSABLE-MEMORY BY ROWSOR COLUMNS”.

FIELD OF THE INVENTION

This invention relates generally to CMOS integrated circuits and moreparticularly to circuits for storing digital data.

SUMMARY OF THE INVENTION

A pair of cross-coupled inverters that hold a digital state are poweredby supplies that also function as row select and column bit lines. Afirst voltage differential between a pair of supply lines for the rowsand a second voltage differential between a pair of supply lines for thecolumns are applied simultaneously to change the state of the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an M×N array of four-transistormemory cells.

FIG. 2 illustrates steps that may be used to read the data in a cell orrow of cells.

FIG. 3 illustrates representative row-supply and column-supply waveformsduring a read of a cell or row of cells.

FIG. 4 illustrates steps that may be used to read the data in a cell orcolumn of cells.

FIG. 5 illustrates representative row-supply and column-supply waveformsduring a read of a cell or column of cells.

FIG. 6 illustrates steps that may be used to write the data in a cell orrow of cells.

FIG. 7 illustrates representative row-supply and column supply waveformsduring a write of a cell or row of cells.

FIG. 8 illustrates steps that may be used to write the data in a cell orcolumn of cells.

FIG. 9 illustrates representative row-supply and column-supply waveformsduring a write of a cell or column of cells.

FIG. 10 illustrates steps that may be used to read and logically OR orAND the data in a column of cells.

FIG. 11 illustrates representative row-supply and column-supplywaveforms during a read and logical OR or AND a column of cells.

FIG. 12 illustrates steps that may be used to read and logically OR orAND the data in a row of cells.

FIG. 13 illustrates representative row-supply and column-supplywaveforms during a read and logical OR or AND a row of cells.

FIG. 14 illustrates steps that may be used to query the columns of anarray of four-transistor memory cells as a content-addressable memory.

FIG. 15 illustrates representative row-supply and column-supplywaveforms to query the columns of an array of four-transistor memorycells as a content addressable memory.

FIG. 16 illustrates steps that may be used to query the rows of an arrayof four-transistor memory cells as a content-addressable memory.

FIG. 17 illustrates representative row-supply and column-supplywaveforms to query the rows of an array of four-transistor memory cellsas a content addressable memory.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic illustration of an array of four-transistor memorycells arranged in M rows and N columns. In this document, thedesignation of rows and columns is arbitrary, may be exchanged, and doesnot imply any particular geometry or arrangement. These designations areused only for illustrative purposes. In FIG. 1, a four-transistor cell100, 120, 130, 140, 144, 146, 150, 154, 156 is shown repeated N times ineach row, and M times in each column. This forms an M×N array that maybe used as a random access memory (RAM).

Four-transistor memory cell 100, which is identical to all the othercells in the M×N array, is comprised of n-channel field effecttransistors (NFETs) 104 and 108 and p-channel field effect transistors(PFETs) 102 and 106. PFET 102 and NFET 104 comprise a first inverterthat is cross-coupled to a second inverter comprising PFET 106 and NFET108. The first inverter has an output node 110 that is connected to thedrains of PFET 102 and NFET 104. The second inverter has an output node112 that is connected to the drains of PFET 106 and NFET 108. The outputnode of the first inverter 110 is connected to the input of the secondinverter (i.e. the gates of PFET 106 and NFET 108). The output node ofthe second inverter 112 is connected to the input of the first inverter(i.e. the gates of PFET 102 and NFET 104). This connection of outputs toinputs forms a cross-coupling of the first and second inverters. Thecross-coupled connection of the first and second inverters of all thecells in the array 100, 120, 130, 140, 144, 146, 150, 154, 156 formcircuits that each store a digital state, or bit.

Using cell 100 as a representative example of the rest of the cells ofthe array, the positive supply voltage for the first inverter of a cellis supplied to the source of PFET 102. Accordingly, the source of PFET102 will also be referred to as the positive supply node for the firstinverter. The negative supply voltage for the first inverter is suppliedto the source of NFET 104. Accordingly, the source of NFET 104 will alsobe referred to as the negative supply node for the first inverter. Thepositive supply voltage for the second inverter is supplied to thesource of PFET 106. Accordingly, the source of PFET 106 will also bereferred to as the positive supply node for the second inverter. Thenegative supply voltage for the second inverter is supplied to thesource of NFET 108. Accordingly, the source of NFET 108 will also bereferred to as the negative supply node for the second inverter. Notethat the first and second inverters of any given cell do not shareeither a positive supply line or a negative supply line.

In the array, the positive supply nodes for the individual first andsecond inverters in a four-transistor cell are connected to separaterow-supply lines. This is shown in FIG. 1 where cells 100, 120, and 130have the positive supply node for their respective first invertersconnected to row-supply line VNROW[0] and the positive supply node fortheir respective second inverters connected to VROW[0]; and where cells140, 144, and 146 have the positive supply node for their respectivefirst inverters connected to VNROW[1] and the positive supply node fortheir respective second inverters connected to VROW[1]; and where cells150, 154, and 156 have the positive supply node for their respectivefirst inverters connected to VNROW[M−11] and the positive supply nodefor their respective second inverters connected to VROW[M−1].

The negative supply lines for the individual first and second invertersin a four-transistor cell are connected to separate column-supply lines.This is shown in FIG. 1 where cells 100, 140, and 150 have the negativesupply node for their respective first inverters connected to VCOL[0]and the negative supply node for their respective second invertersconnected to VNCOL[0]; and where cells 120, 144, and 154 have thenegative supply node for their respective first inverters connected toVCOL[1] and the negative supply node for their respective secondinverters connected to VNCOL[1]; and where cells 130, 146, and 156 havethe negative supply node for their respective first inverters connectedto VCOL[N−1] and the negative supply node for their respective secondinverters connected to VNCOL[N−1].

For a given cell, when the voltages on the positive row-supply lines(i.e. VNROW[0:M−1] and VROW[0:M−1]) and the voltages on the negativecolumn-supply lines (i.e. VCOL[0:N−1] and VNCOL[0:N−1]) are kept at asufficiently large difference from each other (for example, VNROW[0] andVROW[0] at the same voltage but 3.3 volts difference from VCOL[0] andVNCOL[0]) the cross-coupled inverters in each state hold a digitalstate, or bit. However, these voltages may be manipulated to read, orwrite the data in a cell, or group of cells.

FIG. 2 illustrates steps that may be used to read the data in a cell orrow of cells. Cell 100 and its row-supply and column-supply lines willbe used in the discussion of all the figures as a representative cell.However, this is for discussion purposes only and it should beunderstood that all the cells, rows, and columns function substantiallythe same. Also note that in this discussion the row-supply lines VROW[ ]are varied resulting in detectable changes on VCOL[ ] column-supplylines. This is also just for discussion purposes only and it should beunderstood that the VNROW[ ] row-supply lines could have beenmanipulated resulting in detectable changes on the VNCOL[ ]column-supply lines.

In FIG. 2, in a step 202 the voltage is reduced on a selected row-supplyline. For example, to read the cells in the top row of FIG. 1, 100, 120,130, the voltage on row-supply line VROW[0] would be reduced relative tothe voltage on row-supply line VNROW[0] by at least the thresholdvoltage of the PFETs. The other row-supply lines VROW[1:M−1] andVNROW[1:M−1] would all be maintained at a supply-like voltage (forexample, the 3.3V difference from the column-supply lines mentionedabove).

If the first inverter 102, 104 is driving a one (i.e. NFET 104 is offand PFET 102 is on) and the second inverter 106, 108 is driving a zero(i.e. NFET 108 is on and PFET 106 is off) then the reduced voltage onVROW[0] has no effect on cell 100 because the reduced voltage on VROW[0]is not passed from VROW[0] to the output node of the second inverter 112because PFET 106 is off. Accordingly, little or no change in the currentflowing out of VCOL[0] (ICOL[0]) and VNCOL[0] (INCOL[0]) would occur andthese currents ICOL[0] and INCOL[0] would be approximately the same.

In a step 204, this lack of change, or the similar magnitudes of ICOL[0]and INCOL[0], is sensed by a sense circuit connected to at least VCOL[0]to read the state of the inverters in cell 100. For example, if thefirst inverter driving a one is defined as cell 100 holding a zero, thenthis lack of change, or the similar magnitudes of ICOL[0] and INCOL[0],may be detected by a sense circuit which would then output an indicationthat the state of cell 100 was a zero.

If the first inverter 102, 104 is driving a zero (i.e. NFET 104 is onand PFET 102 is off) and the second inverter 106, 108 is driving a one(i.e. NFET 108 is off and PFET 106 is on) then the reduced voltage onVROW[0] is passed from VROW[0] to the output node of the second inverter112 via PFET 106. This causes the voltage between the gate of PFET 102and the source of PFET 102 to exceed the PFET threshold voltage (V_(TP))causing PFET 102 to turn on. As long as the voltage on the output nodeof the second inverter remains an NFET threshold voltage (V_(TN)) aboveVCOL[0], then NFET 104 will remain on. This allows current to flow fromVNROW[0] through PFET 102 and NFET 104 to VCOL[0]. Accordingly, thiscauses a change (increase) in the current flowing out of VCOL[0](ICOL[0]) and but not the current flowing out of VNCOL[0] (INCOL[0]).Therefore the currents ICOL[0] and INCOL[0] would not be approximatelythe same.

In a step 204, this change, or the different magnitudes of ICOL[0] andINCOL[0], is sensed by a sense circuit connected to at least VCOL[0] toread the state of the inverters in cell 100. For example, if the firstinverter driving a zero is defined as cell 100 holding a one, then thechange in ICOL[0], or the difference in magnitudes of ICOL[0] andINCOL[0], may be detected by a sense circuit which would then output anindication that the state of cell 100 was a one.

If ICOL[0] and/or INCOL[0] are passed through an impedance, or allowedto charge a capacitive node, the differences in, or changes to ICOL[0]and INCOL[0] would manifest themselves as voltage differences.Accordingly, the state of a cell may also be detected by sensing avoltage.

FIG. 3 illustrates representative row-supply and column-supply waveformsduring a read of a cell or row of cells. In FIG. 3, VROW[0] starts atits normal operating voltage V_(RNORM). When measuring this voltage withrespect to the column-supply lines VCOL[0:M−1] and VNCOL[0:M−1]V_(RNORM) should be high enough that only one of the FETs in each of thefirst and second inverters in any cell is on at any one time. Thisreduces power consumption and ensures that the current flowing out ofthe column-supply lines ICOL[0:N−1] and INCOL[0:N−1] is minimized.Minimizing the current flowing out of the column supply lines makes itis easier to detect voltage and/or current changes on these lines when acell or multiple cells are being read.

After starting at V_(RNORM) VROW[0] is then shown dropping to a secondvoltage, V_(RREAD1). V_(RREAD1) is a voltage that is low enough to turnon one of the PFETs of a cell when the other row-supply line for thatcell is kept at V_(RNORM). That means that V_(RREAD1) should be at leasta PFET threshold voltage lower that V_(RNORM). After a period of time,VROW[0] is then shown returning from V_(RREAD1) to V_(RNORM). Thecurrent flowing out of column-supply line VCOL[0], ICOL[0] is shown atI_(CSTATIC) at the start when VROW[0] is at V_(RNORM). Then, shortlyafter VROW[0] drops by at least V_(TP), ICOL[0] will either rise toI_(CREAD) (as shown by the solid line) or stay approximately the same(as shown by the dashed line) depending upon the state of cell 100. WhenVROW[0] returns to V_(RNORM), the solid line returns to the I_(CSTATIC)level.

As stated earlier, if ICOL[0] is run through an impedance, or allowed tocharge a capacitance such as is on the column-supply line VCOL[0], itmay produce a detectable voltage change or difference on the VCOL[ ] orVNCOL[ ] lines. One possible voltage waveform is labeled VCOL[0] in FIG.3. This waveform starts at V_(CNORM), and rises to a read level,V_(CREAD1) if ICOL[0] rose to I_(CREAD). VCOL[0] may be clamped to limitits rise to justV_(CREAD1). If ICOL[0] remained at I_(CSTATIC), thenVCOL[0] is shown (by the dashed line) staying at V_(CNORM).

FIG. 4 illustrates steps that may be used to read the data in a cell orcolumn of cells. Note that in this discussion the column-supply linesVCOL[ ] are varied resulting in detectable changes on VROW[ ] row-supplylines. This is also just for discussion purposes only and it should beunderstood that the VNCOL[ ] column-supply lines could have beenmanipulated resulting in detectable changes on the VNROW[ ] row-supplylines.

In FIG. 4, in a step 402 the voltage is increased on a selectedcolumn-supply line. For example, to read the cells in the leftmost handcolumn of FIG. 1, 100, 140, 150, the voltage on column-supply lineVCOL[0] would be increased relative to the voltage on column-supply lineVNCOL[0] by at least the threshold voltage of the NFETs. The othercolumn-supply lines VCOL[1:M−1] and VNCOL[1:M−1] would all be maintainedat a supply-like voltage (for example, ground, or a 3.3V difference fromthe row-supply lines mentioned above).

If the first inverter 102, 104 is driving a one (i.e. NFET 104 is offand PFET 102 is on) and the second inverter 106, 108 is driving a zero(i.e. NFET 108 is on and PFET 106 is off) then the increased voltage onVCOL[0] has no effect because on cell 100 because the increased voltageon VCOL[0] is not passed from VCOL[0] to the output node of the firstinverter 110 because NFET 104 is off. Accordingly, little or no changein the current flowing into VROW[0] (IROW[0]) and VNROW[0] (INROW[0])would occur and these currents IROW[0] and INROW[0] would beapproximately the same.

In a step 404, this lack of change, or the similar magnitudes of IROW[0]and INROW[0], is sensed by a sense circuit connected to at least VROW[0]to read the state of the inverters in cell 100. For example, if thefirst inverter driving a one is defined as cell 100 holding a zero, thenthis lack of change, or the similar magnitudes of IROW[0] and INROW[0],may be detected by a sense circuit which would then output an indicationthat the state of cell 100 was a zero.

If the first inverter 102, 104 is driving a zero (i.e. NFET 104 is onand PFET 102 is off) and the second inverter 106, 108 is driving a one(i.e. NFET 108 is off and PFET 106 is on) then the increased voltage onVCOL[0] is passed from VCOL[0] to the output node of the first inverter110 via NFET 104. This causes the voltage between the gate of NFET 108and the source of NFET 108 to exceed the NFET threshold voltage (V_(TN))causing NFET 108 to turn on. As long as the voltage on the output nodeof the second inverter remains a PFET threshold voltage (V_(TP)) belowVROW[0], then PFET 106 will remain on. This allows current to flow fromVROW[0] through PFET 106 and NFET 108 to VNCOL[0]. Accordingly, thiscauses a change (increase) in the current flowing into VROW[0] (IROW[0])but not the current flowing out of VNROW[0] (INROW[0]). Therefore thecurrents IROW[0] and INROW[0] would not be approximately the same.

In a step 404, this change, or the different magnitudes of IROW[0] andINROW[0], is sensed by a sense circuit connected to at least VROW[0] toread the state of the inverters in cell 100. For example, if the firstinverter driving a zero is defined as cell 100 holding a one, then thechange in IROW[0], or the difference in magnitudes of IROW[0] andINROW[0], may be detected by a sense circuit which would then output anindication that the state of cell 100 was a one.

If IROW[0] and/or INROW [0] are passed through an impedance, or allowedto discharge a capacitive node, the differences in, or changes toIROW[0] and INROW[0] would manifest themselves as voltage differences.Accordingly, the state of a cell may also be detected by sensing avoltage.

FIG. 5 illustrates representative row-supply and column-supply waveformsduring a read of a cell or column of cells. In FIG. 5, VCOL[0] starts atits normal operating voltage V_(CNORM) When measuring this voltage withrespect to the row-supply lines VROW[0:M−1] and VNROW[0:M−1] V_(cNORM)should be low enough that only one of the FETs in each of the first andsecond inverters in any cell is on at any one time. This reduces powerconsumption and ensures that the current flowing into of the row-supplylines IROW[0:N−1] and INROW[0:N−1] is minimized. Minimizing the currentflowing into the row-supply lines makes it is easier to detect voltageand/or current changes on these lines when a cell or multiple cells arebeing read.

After starting at V_(CNORM) VCOL[0] is then shown rising to a secondvoltage, V_(CREAD2) V_(CREAD2) is a voltage that is high enough to turnon one of the NFETs of a cell when the other column-supply line for thatcell is kept at V_(CNORM) That means that V_(CREAD2) should be at leasta NFET threshold voltage higher than V_(CNORM). After a period of time,VCOL[0] is then shown returning from V_(CREAD2) to V_(CNORM). Thecurrent flowing into row-supply line VROW[0], IROW[0] is shown atI_(RSTATIC) at the start when VCOL[0] is at V_(CNORM). Then, shortlyafter VCOL[0] rises by at least V_(TN), IROW[0] will either rise toI_(RREAD) (as shown by the solid line) or stay approximately the same(as shown by the dashed line) depending upon the state of cell 100. WhenVCOL[0] returns to V_(CNORM), the solid line returns to the I_(RSTATIC)level.

As stated earlier, if IROW[0] is run through an impedance, or allowed todischarge a capacitance such as is on the row-supply line VROWL[0], itmay produce a detectable voltage change or difference on the VROW[ ] orVNROW[ ] lines. One possible voltage waveform is labeled VROW[0] in FIG.5. This waveform starts at V_(RNORM), and drops to a read level,V_(RREAD2) if IROW[0] rose to I_(RREAD). VROWL[0] may be clamped tolimit its fall to just V_(RREAD1). If IROW[0] remained at I_(RSTATIC),then VROW[0] is shown (by the dashed line) staying at V_(RNORM)

To read a row of cells, either a VROW or VNROW line is reduced involtage and the current or voltage on the VCOL or VNCOL lines ismeasured to detect the state of each cell in the row of cells addressedby the reduced VROW or VNROW line. To read a column of cells, either aVCOL or VNCOL line is increased in voltage and the current or voltage onthe VROW or VNROW lines is measured to detect the state of each cell inthe column of cells addressed by the increased VCOL or VNCOL line. Notethat the process for accessing a row of cells and for accessing a columnof cells, in the array itself, require the same hardware. This is unlikeconventional six-transistor memory arrays that have certain transistorsin each cell dedicated to accessing either a column, or a row, but notboth. Accordingly, the array of four-transistor memory cells may beaccessed on either a column or row basis.

FIG. 6 illustrates steps that may be used to write the data in a cell orrow of cells. In FIG. 6, in a step 602 the voltage is reduced on aselected row-supply line. For example, to write a first state (i.e. thefirst inverter 102, 104 driving a one and the second inverter driving azero) into some or all of the cells in the top row of FIG. 1, 100, 120,130, the voltage on row-supply line VROW[0] would be reduced relative tothe voltage on row-supply line VNROW[0] by at least the thresholdvoltage of the PFETs. The other (non-selected) row-supply linesVROW[1:M−1] and VNROW[1:M−1] would all be maintained at a supply-likevoltage (for example, the 3.3V difference from the column-supply linesmentioned above).

Using cell 100 as an example, if the first inverter 102, 104 is drivinga zero (i.e. NFET 104 is on and PFET 102 is off) and the second inverter106, 108 is driving a one (i.e. NFET 108 is off and PFET 106 is on) thenthe reduced voltage on VROW[0] is passed from VROW[0] to the output nodeof the second inverter 112 via PFET 106. This causes the voltage betweenthe gate of PFET 102 and the source of PFET 102 to exceed the PFETthreshold voltage (V_(TP)) causing PFET 102 to turn on. PFET 102 turningon causes the voltage on the output node of the first inverter 110 torise. In the case of a read of the cell, above, this voltage is notallowed to rise enough to flip the state of the cell. However, in thecase of a write, the voltage rise on node 110 is increased by raisingthe voltage, or allowing the voltage to raise, in a step 604 on theappropriate VCOL[ ] lines (VCOL[0] is the appropriate line to write thisfirst state into cell 100, VCOL[1] for cell 120, etc.) such that thevoltage on the output node of the first inverter 110 exceeds thetrip-point of the second inverter. This flips the state of the cell intoa second state where the second inverter is now driving a zero and thefirst inverter is now driving a one. Note that the voltage increase onthe VCOL[ ] lines does not need to be extraordinarily large because thetrip point of the second inverter has been reduced due to its reducedsupply voltage and because PFET 102 is fighting against NFET 104 andraising the VCOL[ ] line decreases the ability of NFET 104 to win thatfight. In the non-selected rows, these factors are not present so theyare not affected by the rise on the VCOL[ ] line or lines. Note alsothat if, for a particular cell or group of cells in the selected row, itis not desired to set them in this first state, step 604 is notperformed on those cells and they will maintain their original value aswill all the cells in the non-selected rows.

If the first inverter 102, 104 were already driving a one (i.e. NFET 104is off and PFET 102 is on) and the second inverter 106, 108 is driving azero (i.e. NFET 108 is on and PFET 106 is off) then the reduced voltageon VROW[0] has no effect on cell 100 because the reduced voltage onVROW[0] is not passed from VROW[0] to the output node of the secondinverter 112 because PFET 106 is off and the increased voltage onVCOL[0] is not passed to the output node of the first inverter 110because NFET 104 is also off.

In order to write a second state (i.e. the first inverter 102, 104driving a zero and the second inverter driving a one) into some or allof the cells in the top row of FIG. 1, 100, 120, 130, the processdescribed above is followed except that the voltage on row-supply lineVNROW[0] would be reduced relative to the voltage on row-supply lineVROW[0] (instead of the other way around) by at least the thresholdvoltage of the PFETs in step 602. The other (non-selected) row-supplylines VROW[1:M−1] and VNROW[1:M−1] would still all be maintained at asupply-like voltage. Then in step 604, the column-supply line that israised, or allowed to rise, in order to set the cell 100 in this secondstate would be VNCOL[0] (instead of VCOL[0]).

Since different row-supply and column-supply lines need to bemanipulated to write the first state and second state into cells in thesame row, a two-step process may be used to write arbitrary data intomultiple cells of a row. To write this arbitrary data, all the cellsthat are to contain one state (for example, the first state) arewritten, then all the cells that are to contain the other state (forexample, the second state) are written. The advantage of this two-stepprocess is that it allows arbitrary data to be written into onlyportions of a row. For example, to write only one byte (or nibble, orword, or arbitrary bits, etc.), the two step process would be performedonly on the cells of that byte. The column-supply lines for the othercells would not be manipulated and would therefore retain their originalvalues. This is unlike many conventional RAM array that must performwrite operations on whole rows.

FIG. 7 illustrates representative row-supply and column-supply waveformsduring a write of a cell or row of cells. In FIG. 7, the row-supplyline, VROW[0] starts at V_(RNORM) and the column-supply line VCOL[0]starts at V_(CNORM). Then, to write the cell, VROW[0] is lowered toV_(ROWWR1) and VCOL[0] rises to V_(COLWR1). This is shown by the solidlines in FIG. 7. The other row-supply line VNROW[0] is kept at V_(RNORM)during the write and the other column-supply line is kept at V_(CNORM)during the write. Therefore, these are not shown. The exact timing ofthe lowering of VROW[0] and the rising of VCOL[0] is not critical aslong as there is a long enough period time where VROW[0] is lowered andVCOL[0] is raised to set the cell in the desired state. The dashed linefor VCOL[0] that stays at V_(CNORM) illustrates the waveform used whenthis cell is not to be set in this state. As drawn, this figureillustrates waveforms to set a cell in a first state, to set the cell ina second state, VNROW[0] would be substituted for VROW[0] and VNCOL[0]would be substituted for VCOL[0] and VROW[0] and VNCOL[0] would be keptat V_(RNORM) and V_(CNORM), respectively.

FIG. 8 illustrates steps that may be used to write the data in a cell orcolumn of cells. In FIG. 8, in a step 802 the voltage is increased on aselected column-supply line. For example, to write a first state (i.e.the first inverter 102, 104 driving a one and the second inverter106,108 driving a zero) into some or all of the cells in the leftmostcolumn of FIG. 1, 100, 140, 150, the voltage on column-supply lineVCOL[0] would be increased relative to the voltage on row-supply lineVNCOL[0] by at least the threshold voltage of the NFETs. The other(non-selected) column-supply lines VCOL[1:M−1] and VNCOL[1:M−1] wouldall be maintained at a supply-like voltage (for example, the 3.3Vdifference from the row-supply lines mentioned above).

Using cell 100 as an example, if the first inverter 102, 104 is drivinga zero (i.e. NFET 104 is on and PFET 102 is off) and the second inverter106, 108 is driving a one (i.e. NFET 108 is off and PFET 106 is on) thenthe increased voltage on VCOL[0] is passed from VCOL[0] to the outputnode of the first inverter 110 via NFET 104. This causes the voltagebetween the gate of NFET 108 and the source of NFET 108 to exceed theNFET threshold voltage (V_(TN)) causing NPFET 108 to turn on. NFET 108turning on causes the voltage on the output node of the second inverter112 to fall. In the case of a read of the cell, above, this voltage isnot allowed to fall enough to flip the state of the cell. However, inthe case of a write, the magnitude of the voltage decrease on node 112is increased by reducing the voltage, or allowing the voltage to fall,in a step 804 on the appropriate VROW[ ] lines (VROW[0] is theappropriate line to write this first state into cell 100, VROW[1] forcell 140, etc.) such that the voltage on the output node of the secondinverter 112 drops below the trip-point of the first inverter. Thisflips the state of the cell into a second state where the secondinverter is now driving a zero and the first inverter is now driving aone. Note that the voltage decrease on the VROW[ ] lines does not needto be extraordinarily large because the trip point of the first inverterhas been increased due to the increase on its negative supply voltageand because PFET 106 is fighting against NFET 108 and lowering the VROW[] line decreases the ability of PFET 106 to win that fight. In thenon-selected rows, these factors are not present so they are notaffected by the rise on the VCOL[ ] line or lines. Note also that if,for a particular cell or group of cells in the selected column, it isnot desired to set them in this first state, step 804 is not performedon those cells and they will maintain their original value as will allthe cells in the non-selected columns.

If the first inverter 102, 104 were already driving a one (i.e. NFET 104is off and PFET 102 is on) and the second inverter 106, 108 is driving azero (i.e. NFET 108 is on and PFET 106 is off) then the increasedvoltage on VCOL[0] has no effect on cell 100 because the increasedvoltage on VCOL[0] is not passed from VCOL[0] to the output node of thefirst inverter 110 because NFET 104 is off and the increased voltage onVROW[0] is not passed to the output node of the second inverter 112because PFET 106 is also off.

In order to write a second state (i.e. the first inverter 102, 104driving a zero and the second inverter 106, 108 driving a one) into someor all of the cells in the leftmost column of FIG. 1, 100, 140, 150, theprocess described above is followed except that the voltage oncolumn-supply line VNCOL[0] would be increased relative to the voltageon column-supply line VCOL[0] (instead of the other way around) by atleast the threshold voltage of the NFETs in step 802. The other(non-selected) column-supply lines VCOL[1:M−1] and VNCOL[1:M−1] wouldstill all be maintained at a supply-like voltage. Then in step 804, therow-supply line that is lowered, or allowed to fall, in order to set thecell 100 in this second state would be VNROW[0] (instead of VROW[0]).

Since different row-supply and column-supply lines need to bemanipulated to write the first state and second state into cells in thesame column, a two-step process may be used to write arbitrary data intomultiple cells of a column. To write this arbitrary data, all the cellsthat are to contain one state (for example, the first state) arewritten, then all the cell that are to contain the other state (forexample, the second state) are written. The advantage of this two-stepprocess is that it allows arbitrary data to be written into onlyportions of a column. For example, to write only one byte (or nibble, orword, or arbitrary bits, etc.), the two step process would be performedonly on the cells of that byte. The row-supply lines for the other cellswould not be manipulated and would therefore retain their originalvalues. This is unlike many conventional RAM arrays.

FIG. 9 illustrates representative row-supply and column-supply waveformsduring a write of a cell or column of cells. In FIG. 9, thecolumn-supply line, VCOL[0] starts at V_(CNORM) and the row-supply lineVROWL[0] starts at V_(RNORM). Then, to write the cell, VCOL[0] is raisedto V_(COLWR2) and VROW[0] decreases to V_(ROWWR2). This is shown by thesolid lines in FIG. 9. The other column-supply line VNCOL[0] is kept atV_(CNORM) during the write and the other row-supply line is kept atV_(RNORM) during the write. Therefore, these are not shown. The exacttiming of the increasing of VCOL[0] and the falling of VROW[0] is notcritical as long as there is a long enough period time where VCOL[0] israised and VROW[0] is lowered to set the cell in the desired state. Thedashed line for VROW[0] that stays at V_(RNORM) illustrates the waveformused when this cell is not to be set in this state. As drawn, thisfigure illustrates waveforms to set a cell in a first state, to set thecell in a second state, VNCOL[0] would be substituted for VCOL[0] andVNROW[0] would be substituted for VROW[0] and VCOL[0] and VNROW[0] wouldbe kept at V_(CNORM) and V_(RNORM), respectively.

FIG. 10 illustrates steps that may be used to read and logically OR orAND the data in a column of cells. In FIG. 10, in a step 1002 thevoltage is reduced on multiple selected row-supply lines. For example,to logically OR the contents of the cells in the leftmost column of FIG.1, 100, and 140, the voltage on row-supply line VROW[0] and VROW[1]would be reduced relative to the voltage on row-supply lines VNROW[0]and VNROW[1], respectively, by at least the threshold voltage of thePFETs. The other row-supply lines VROW[2:M−1] and VNROW[2:M−1] would allbe maintained at a supply-like voltage (for example, the 3.3V differencefrom the column-supply lines mentioned above). As in a regular row read(detailed in the discussions of FIG. 2 and FIG. 3) if cell 100 is in astate where the first inverter 102, 104 is driving a zero, then loweringVROW[0] causes current to flow from VNROW[0] to VCOL[0] causing anincrease in the current ICOL[0]. Likewise, if cell 140 is in a statewhere the first inverter 102, 104 is driving a zero, then loweringVROW[1] causes current to flow from VNROW[1] to VCOL[0] also causing anincrease in the current ICOL[0]. Since these two currents are in thesame direction, they are additive. Therefore, only if both of the cells100 and 140 are in a state where their first inverters are driving oneswill there not be an increase in the current ICOL[0]. If the state wherethe first inverters driving ones is defined as the cell holding a one,and the current ICOL[0] not increasing is defined as a logical oneoutput state, and the increased current ICOL[0] defined as a logicalzero output state, then the current ICOL[0] will reflect the logical ANDof the state of all the selected cells in the leftmost column (in thisexample, 100 and 140).

This process works just as well for the other columns. A set of rows isselected, and the columns output the logical AND of the contents of thecells in that column that were selected. This is called the logical AND,by column, of the selected rows. Similarly, using the same definitionfor when a cell is holding a one (i.e. the first inverter driving a one)the logical OR, by column, of the contents of cells in a set of selectedrows may be computed by using the VNROW[ ] lines to select rows andexamining the current INCOL[ ].

Also, if the state where the first inverters driving ones is defined asthe cell holding a zero, and the current ICOL[0] not increasing isdefined as a logical zero output state, and the increased currentICOL[0] defined as a logical one output state, then the current ICOL[0]will reflect the logical OR of the state of all the selected cells inthe leftmost column (in this example, 100 and 140). In other words, thisset of definitions results in the logical OR, by column, of the selectedrows but uses the VROW[ ] lines for selection. Likewise, using the samedefinition for when a cell is holding a zero (i.e. the first inverterdriving a one) the logical AND of selected rows may be computed by usingthe VNROW[ ] lines to select rows and examining the current INCOL[0].Other combinations involving the definition of which states of the cellsrepresent which logical value (i.e. “0” or “1”), the currents ICOL[ ]and INCOL[ ], and which lines (VROW[ ] or VNROW[ ]) are used to selectthe rows may be constructed to determine other logical functions such asNAND and NOR of the contents of the cells in a column.

FIG. 11 illustrates representative row-supply and column-supplywaveforms during a read and logical OR or AND a column of cells. In FIG.11, VROW[0] and VROW[1] start at their normal operating voltageV_(RNORM). After starting at V_(RNORM) VROW[0] and VROW[1] are thenshown dropping to a second voltage, V_(RREAD1). V_(RREAD1) is a voltagethat is low enough to turn on one of the PFETs of a cell when the otherrow-supply line for that cell is kept at V_(RNORM). That means thatV_(RREAD1) should be at least a PFET threshold voltage lower thatV_(RNORM). After a period of time, VROW[0] and VROW[1] are then shownreturning from V_(RREAD1) to V_(RNORM). The exact timing of the dropsand rises of VROW[0] and VROW[1] relative to each other is not criticalas long as they are both at their reduced voltage levels for some periodof time long enough to read either the voltage or current on the VCOL[ ]or VNCOL[ ] lines. The current flowing out of column-supply lineVCOL[0], ICOL[0] is shown at I_(CSTATIC) at the start when both VROW[0]and VROW[1] are at V_(RNORM). Then, shortly after VROW[0] and VROW[1]drop by at least V_(TP), ICOL[0] will either rise to at least I_(CREAD)(when, because of their states, only one cell is dumping current ontothe column-supply line as represented by the solid line 1104), riseabove I_(CREAD) (when, because of their states, more than one cell isdumping current onto the column-supply line as represented by dashedline 1106) or stay approximately the same (when, because of theirstates, no cells are dumping non-static current onto the column-supplyline as represented by dashed line 1102) depending upon the state ofcells 100 and 140. When VROW[0] and VROW[1] return to V_(RNORM), thelines 1104 and 1106 return to the I_(CSTATIC) level.

As stated earlier, if ICOL[0] is run through an impedance, or allowed tocharge a capacitance such as is on the column-supply line VCOL[0], itmay produce a detectable voltage change or difference on the VCOL[ ] orVNCOL[ ] lines. This also true when performing logic functions on thecontents of the cells in a column. One possible set of voltage waveformsis labeled VCOL[0] in FIG. 11. These waveforms starts at V_(CNORM). IfICOL[0] rose to I_(CREAD) waveforms similar to 1110 and 1112 may befollowed with VCOL[0] rising to V_(CRFAD1). Waveform 1112 rises fasterto V_(CREAD) than waveform 1110 and is intended to represent the casewhen more than one cell in the column is dumping current onto thecolumn-supply line, such as is shown with waveform 1106. VCOL[0] may beclamped to limit its rise to justV_(CREAD1). If ICOL[0] remained atI_(CSTATIC), then VCOL[0] is shown (by the dashed line 1108) staying atV_(CNORM),

FIG. 12 illustrates steps that may be used to read and logical OR or ANDthe data in a row of cells. In FIG. 12, in a step 1202 the voltage isincreased on multiple selected column-supply lines. For example, tologically OR the contents of the cells in the top row of FIG. 1, 100,and 120, the voltage on column-supply line VCOL[0] and VCOL[1] would beincreased relative to the voltage on column-supply lines VNCOL[0] andVNCOL[1], respectively, by at least the threshold voltage of the NFETs.The other column-supply lines VCOL[2:M−1] and VNCOL[2:M−1] would all bemaintained at a supply-like voltage (for example, the 3.3V differencefrom the row-supply lines mentioned above). As in a regular column read(detailed in the discussions of FIG. 4 and FIG. 5) if cell 100 is in astate where the first inverter 102, 104 is driving a zero, then raisingVCOL[0] causes current to flow from VROW[0] to VNCOL[0] causing anincrease in the current IROW[0]. Likewise, if cell 140 is in a statewhere the first inverter 102, 104 is driving a zero, then raisingVCOL[1] causes current to flow from VROW[0] to VNCOL[1] also causing anincrease in the current IROW[0]. Since these two currents are in thesame direction, they are additive. Therefore, only if both of the cells100 and 120 are in a state where their first inverters are driving oneswill there not be an increase in the current IROW[0]. If the state wherethe first inverters driving ones is defined as the cell holding a one,and the current IROW[0] not increasing is defined as a logical oneoutput state, and the increased current IROW[0] defined as a logicalzero output state, then the current IROW[0] will reflect the logical ANDof the state of all the selected cells in the topmost row (in thisexample, 100 and 120).

This process works just as well for the other rows. A set of columns isselected, and the rows output the logical AND of the contents of thecells in that row that were selected. This is called the logical AND, byrow, of the selected columns. Similarly, using the same definition forwhen a cell is holding a one (i.e. the first inverter driving a one) thelogical OR, by row, of the contents of cells in a set of selectedcolumns may be computed by using the VNCOL[ ] lines to select columnsand examining the current INROW[ ].

Also, if the state where the first inverters driving ones is defined asthe cell holding a zero, and the current IROW[0] not increasing isdefined as a logical zero output state, and the increased currentIROW[0] defined as a logical one output state, then the current IROW[0]will reflect the logical OR of the state of all the selected cells inthe topmost row (in this example, 100 and 120). In other words, this setof definitions results in the logical OR, by row, of the selectedcolumns but uses the VCOL[ ] lines for selection. Likewise, using thesame definition for when a cell is holding a zero (i.e. the firstinverter driving a one) the logical AND of selected columns may becomputed by using the VNCOL[ ] lines to select columns and examining thecurrent INROW[0]. Other combinations involving the definition of whichstates of the cells represent which logical value (i.e. “0” or “1”), thecurrents IROW[ ] and INROW[ ] and which lines (VCOL[ ] or VNCOL[ ]) areused to select the columns may be constructed to determine other logicalfunctions such as NAND and NOR of the contents of the cells in a column.

FIG. 13 illustrates representative row-supply and column-supplywaveforms during a read and logical OR or AND a row of cells. In FIG.13, VCOL[0] and VCOL[1] start at their normal operating voltageV_(CNORM). After starting at V_(CNORM) VCOL[0] and VCOL[1] are thenshown rising to a second voltage, V_(CREAD1). V_(CREAD1) is a voltagethat is high enough to turn on one of the NFETs of a cell when the othercolumn-supply line for that cell is kept at V_(CNORM). That means thatV_(CRELAD1) should be at least a NFET threshold voltage lower thatV_(CNORM). After a period of time, VCOL[0]and VCOL[1] are then shownreturning from V_(CREAD1) to V_(CNORM). The exact timing of the dropsand rises of VCOL[0] and VCOL[1] relative to each other is not criticalas long as they are both at their increased voltage levels for someperiod of time long enough to read either the voltage or current on theVROW[ ] or VNROW[ ] lines. The current flowing into row-supply lineVROW[0], IROW[0] is shown at I_(RSTATIC) at the start when both VCOL[0]and VCOL[1] are at V_(CNORM) Then, shortly after VCOL[0] and VCOL[1]rise by at least V_(TN), IROW[0] will either rise to at least I_(RREAD)(when, because of their states, only one cell is pulling current fromthe row-supply line as represented by the solid line 1304), rise aboveI_(RREAD) (when, because of their states, more than one cell is pullingcurrent from the row-supply line as represented by dashed line 1306), orstay approximately the same (when, because of their states, no cells arepulling non-static current from the row-supply line as represented bydashed line 1302) depending upon the state of cells 100 and 120. WhenVCOL[0] and VCOL[1] return to V_(CNORM), the lines 1304 and 1306 returnto the I_(RSTATIC) level.

As stated earlier, if IROW[0] is run through an impedance, or allowed todischarge a capacitance such as is on the row-supply line VROW[0], itmay produce a detectable voltage change or difference on the VROW[ ] orVNROW[ ] lines. This is also true when performing logic functions on thecontents of the cells in a row. One possible set of voltage waveforms islabeled VROW[0] in FIG. 13. These waveforms starts at V_(RNORM). IfIROW[0] rose to I_(RREAD) waveforms similar to 1310 and 1312 may befollowed with VCOL[0] dropping to V_(RREAD1). Waveform 1312 drops fasterto V_(RREAD1) than waveform 1310 and is intended to represent the casewhen more than one cell in the row is pulling current from therow-supply line, such as is shown with waveform 1306. VROW[0] may beclamped to limit its drop to justV_(RREAD1). If IROW[0] remained atI_(RSTATIC), then VROW[0] is shown (by the dashed line 1308) staying atV_(RNORM).

FIG. 14 illustrates steps that may be used to query the columns of anarray of four-transistor memory cells as a content-addressable memory.In FIG. 14, in a step 1402 row-supply lines are selected for voltagereduction based upon the query pattern desired. For each cell in acolumn to be queried, one of VROW[ ] or VNROW[ ] is selected dependingupon whether a zero or one state in the cell should result in a match.For example, if the first inverter driving a zero is defined as cell 100holding a one, then the row-supply line selected for a particular row ofcells would be the VNROW[ ] to look for matches to that one state. Tolook for matches to the zero state (i.e. the first inverter driving aone), the VROW[ ] line would be selected. If it is desired not to querya row or rows (i.e. not include a row or rows of cells in the patternmatch), then neither VROW[ ] or VNROW[ ] for those rows would beselected.

In a step 1404, the voltage is reduced on a selected row-supply lines.For example, to query the cells in the top row of FIG. 1, 100, 120, 130,for the state where the first inverter is driving a one, and to querythe cells in the second row 140, 144, 146 of FIG. 1 for the state wherethe first inverter is driving a zero, the voltage on row-supply lineVROW[0] would be reduced relative to the voltage on row-supply lineVNROW[0] by at least the threshold voltage of the PFETs and VNROW[1]would be reduced relative to the voltage on row-supply line VROW[1]. Inother words, VROW[0] and VNROW[1] were the row-supply lines selected inthe previous step. The other row-supply lines VROW[2:M−1] andVNROW[2:M−1] would be maintained at a supply-like voltage (for example,the 3.3V difference from the column-supply lines mentioned above)provided they are not part of the desired query.

If the contents of the queried cells in a given column match the query,then neither VCOL[ ] or VNCOL[ ] line will show an increase in current(i.e. ICOL[ ] or INCOL[ ]) or voltage. Using the above as an example, ifcell 100 has its first inverter driving a one and cell 140 has its firstinverter driving a zero, and VROW[0] and VNROW[1] are the selectedrow-supply lines (from step 1402), then the reduced voltages on VROW[0]and VNROW[1] will not cause a significant increase in ICOL[0] orINCOL[0] (or a voltage increase on VCOL[0] or VNCOL[0]). Accordingly,this lack of increase on both column-supply lines for a given columnindicates that the query has matched the contents of the cells in thatcolumn. Unlike a regular read, logical OR, or logical AND, since bothcolumn-supply lines must show this lack of increase in current (orvoltage) it is necessary to sense both column-supply lines to see ifthere was a match to the query.

If the contents of the queried cell in a given column do not match thequery, then one or both of VCOL[ ] or VNCOL[ ] lines will show anincrease in current (i.e. ICOL[ ] or INCOL[ ]) or voltage. Using thesame query above as an example, if cell 120 has its first inverterdriving a one, and VROW[0] is a selected row-supply line (from step1402), then the reduced voltage on VROW[0] will cause a detectableincrease in ICOL[1] (or voltage increase on VCOL[1]). This increaseindicates that the query did not match the contents of at least one cellin that column (i.e. in this case, at least the contents of cell 120 didnot match). Likewise, if cell 144 has its first inverter driving a zero,and VNROW[ ] is a selected row-supply line (from step 1402), then thereduced voltage on VNROW[1] will cause a detectable increase in INCOL[1](or voltage increase on VNCOL[1]. This increase indicates that the querydid not match the contents of at least one cell in that column (i.e. inthis case, at least the content of cell 144 did not match). If more thanone cell in a column does not match the query, it is possible for bothcolumn-supply lines for that column to show a detectable increase incurrent.

FIG. 15 illustrates representative row-supply and column-supplywaveforms to query the columns of an array of four-transistor memorycells as a content addressable memory. In FIG. 15, VROW[0], VNROW[0],VROW[1] and VNROW[1] start at their normal operating voltage V_(RNORM).After starting at V_(RNORM) VROW[0] and VNROW[1] are then shown droppingto a second voltage, V_(RREAD1). V_(RREAD1) is a voltage that is lowenough to turn on one of the PFETs of a cell when the other row-supplyline for that cell is kept at V_(RNORM). That means that V_(RREAD1)should be at least a PFET threshold voltage lower that V_(RNORM). Aftera period of time, VROW[0] and VNROW[1] are then shown returning fromV_(RREAD1) to V_(RNORM). The exact timing of the drops and rises ofVROW[0] and VNROW[ 1] relative to each other is not critical as long asthey are both at their reduced voltage levels for some period of timelong enough to read either the voltage or current on the VCOL[ ] andVNCOL[ ]lines. The current flowing out of column-supply line VCOL[0],ICOL[0] and VNCOL[0], INCOL[0] is shown at I_(CSTATIC) at the start whenboth VROW[0] and VROW[1] are at V_(RNORM). Then, shortly after VROW[0]and VNROW[1] drop by at least V_(TP), ICOL[0] and INCOL[0] will eitherrise to at least I_(CREAD) (when, because of their states, only one cellis dumping current onto the column-supply line as represented by thesolid lines 1506 and 1508), rise above I_(CREAD) (when, because of theirstates, more than one cell is dumping current onto the column-supplyline) or stay approximately the same (when, because of their states, nocells are dumping non-static current onto the column-supply line asrepresented by dashed lines 1502 and 1504) depending upon the state ofcells 100 and 140. When VROW[0] and VNROW[1] return to V_(RNORM), thelines 1504 and 1506 return to the I_(CSTATIC) level. Dashed lines 1502and 1504 represent what happens when the query and the contents of thecells match.

FIG. 16 illustrates steps that may be used to query the rows of an arrayof four-transistor memory cells as a content-addressable memory. In FIG.16, in a step 1602 column-supply lines are selected for increasedvoltage based upon the query pattern desired. For each cell in a row tobe queried, one of VCOL[ ] or VNCOL[ ] is selected depending uponwhether a zero or one state in the cell should result in a match. Forexample, if the first inverter driving a zero is defined as cell 100holding a one, then the column-supply line selected for a particular rowof cells would be the VNCOL[ ] to look for matches to that one state. Tolook for matches to the zero state (i.e. the first inverter driving aone), the VCOL[ ] line would be selected. If it is desired not to querya column or columns (i.e. not include a column or columns of cells inthe pattern match), then neither VCOL[ ] or VNCOL[ ] for those rowswould be selected.

In a step 1604, the voltage is increased on the selected column-supplylines. For example, to query the cells in the leftmost column of FIG. 1,100, 140, 150, for the state where the first inverter is driving a one,and to query the cells in the second column 120, 144, 154 of FIG. 1 forthe state where the first inverter is driving a zero, the voltage oncolumn-supply line VCOL[0] would be increased relative to the voltage oncolumn-supply line VNCOL[0] by at least the threshold voltage of theNFETs and VNCOL[1] would be increased relative to the voltage oncolumn-supply line VCOL[1]. In other words, VCOL[0] and VNCOL[1] werethe column-supply lines selected in the previous step. The othercolumn-supply lines VCOL[2:N−1] and VNCOL[2:N−1] would be maintained ata supply-like voltage (for example, the 3.3V difference from therow-supply lines mentioned above) provided they are not part of thedesired query.

If the contents of the queried cells in a given row match the query,then neither VROW[0] or VNROW[0] line will show an increase in current(i.e. IROW[0] or INROW[0]) or decrease in voltage. Using the above as anexample, if cell 100 has its first inverter driving a one and cell 120has its first inverter driving a zero, and VCOL[0] and VNCOL[1] are theselected column-supply lines (from step 1602), then the increasedvoltages on VCOL[0] and VNCOL[1] will not cause a significant increasein IROW[0] or INROW[0] (or a voltage decrease on VROW[0] or VNROW[0]).Accordingly, this lack of increased current on both row-supply lines fora given row indicates that the query has matched the contents of thecells in that row. Unlike a regular read, logical OR, or logical AND,since both row-supply lines must show this lack of increase in current(or decrease in voltage) it is necessary to sense both row-supply linesto see if there was a match to the query.

If the contents of the queried cell in a given row do not match thequery, then one or both of VROW[0] or VNROW[0] lines will show anincrease in current (i.e. IROW[0] or INROW[0]) or decrease in voltage.Using the same query above as an example, if cell 140 has its firstinverter driving a one, and VCOL[0] is a selected column-supply line(from step 1602), then the increased voltage on VCOL[0] will cause adetectable increase in IROW[1] (or voltage decrease on VROW[1]). Thisincrease indicates that the query did not match the contents of at leastone cell in that row (i.e. in this case, at least the contents of cell140 did not match). Likewise, if cell 144 has its first inverter drivinga zero, and VNCOL[1] is a selected column-supply line (from step 1602),then the increased voltage on VNCOL[1] will cause a detectable increasein INROW[1] (or voltage decrease on VNROW[1]). This increase (or voltagedecrease) indicates that the query did not match the contents of atleast one cell in that row (i.e. in this case, at least the content ofcell 144 did not match). If more than one cell in a row does not matchthe query, it is possible for both row-supply lines for that row to showa detectable increase in current.

FIG. 17 illustrates representative row-supply and column-supplywaveforms to query the rows of an array of four-transistor memory cellsas a content addressable memory. In FIG. 17, VCOL[0], VNCOL[0], VCOL[1]and VNCOL[1] start at their normal operating voltage V_(CNORM). Afterstarting at V_(CNORM) VCOL[0] and VNCOL[1] are then shown rising to asecond voltage, V_(CREAD1). V_(CREAD1) is a voltage that is high enoughto turn on one of the NFETs of a cell when the other column-supply linefor that cell is kept at V_(CNORM). That means that V_(CREAD1) should beat least a NFET threshold voltage higher that V_(CNORM). After a periodof time, VCOL[0] and VNCOL[1] are then shown returning from V_(CREAD1)to V_(CNORM). The exact timing of the drops and rises of VCOL[0] andVNCOL[1] relative to each other is not critical as long as they are bothat their increased voltage levels for some period of time long enough toread either the voltage or current on the VROW[0] and VNROW[0] lines.The current flowing into the row-supply line VROW[0], IROW[0], and thecurrent flowing into the row-supply line VNROW[0], INROW[0] is shown atI_(RSTATIC) at the start when both VCOL[0] and VCOL[1] are at V_(CNORM).Then, shortly after VCOL[0] and VNCOL[1] rise by at least V_(TN),IROW[0] and INROW[0] will either rise to at least I_(RREAD) (when,because of their states, only one cell is pulling current from therow-supply line as represented by the solid lines 1706 and 1708), riseabove I_(RREAD) (when, because of their states, more than one cell ispulling current from the row-supply line), or stay approximately thesame (when, because of their states, no cells are pulling non-staticcurrent from the row-supply line as represented by dashed lines 1702 and1704) depending upon the state of cells 100 and 120. When VCOL[0] andVNCOL[1] return to V_(CNORM), the lines 1704 and 1706 return to theI_(RSTATIC) level. Dashed lines 1702 and 1704 represent what happenswhen the query and the contents of the cells in the row match.

What is claimed is:
 1. A method of writing a memory array, comprising:creating a first voltage differential between a first row-supply lineand a second row-supply line; and, creating a second voltagedifferential between a first column-supply line and a secondcolumn-supply line.
 2. The method of claim 1 wherein said first voltagedifferential is at least a first threshold voltage of a first type offield-effect transistor in magnitude and said second voltagedifferential is less than said a second threshold voltage of a secondtype of field-effect transistor in magnitude.
 3. The method of claim 1wherein said first row-supply line and said first column-supply line areconnected to a first inverter and said first voltage differentialcomprises said first row-supply line being at a greater voltage thansaid second row-supply line and said second voltage differentialcomprises said first column-supply line being at a greater voltage thansaid second colunm-supply line.
 4. The method of claim 3, furthercomprising: creating a third voltage differential between said firstrow-supply line and said second row-supply line; and, creating a fourthvoltage differential between said first column-supply line and saidsecond column-supply line, wherein said third voltage differentialcomprises said second row-supply line being at a greater voltage thansaid first row-supply line and said fourth voltage differentialcomprises said second column-supply line being at a greater voltage thansaid first column-supply line.
 5. The method of claim 1, furthercomprising: creating a third voltage differential between said firstrow-supply line and said second row-supply line; and, creating a fourthvoltage differential between said first column-supply line and saidsecond column-supply line wherein said third voltage differential isopposite in polarity from said first voltage differential and saidfourth voltage differential is opposite in polarity from said secondvoltage differential.
 6. The method of claim 5 wherein said firstvoltage differential is at least a first threshold voltage of a firsttype of field-effect transistor in magnitude and said second voltagedifferential is less than said a second threshold voltage of a secondtype of field-effect transistor in magnitude.
 7. A method of writing amemory array, comprising: driving a first voltage differential between afirst supply line and a second supply line wherein said first supplyline is connected to a first inverter in a plurality cells of a row andsaid second supply line is connected to a second inverter in a pluralityof cells of said row thereby causing a first current to flow betweensaid first supply line and at least a third supply line when a cell ofsaid plurality of cells in said row is in a first state; and, driving asecond voltage differential between said third supply line and a fourthsupply line thereby causing said cell to switch between said first stateand a second state and wherein said third supply line is connected tosaid first inverter and said fourth supply is connected to said secondinverter.
 8. The method of claim 7, wherein said step of driving a firstvoltage differential comprises decreasing a voltage on said secondsupply line relative to said first supply line.
 9. The method of claim7, wherein said step of driving a second voltage differential comprisesincreasing a voltage on said third supply line relative to said fourthsupply line.
 10. The method of claim 9, wherein said step of driving afirst voltage differential comprises decreasing a voltage on said secondsupply line relative to said first supply line.
 11. The method of claim9, wherein said step of driving a first voltage differential comprisesincreasing a voltage on said first supply line relative to said secondsupply line.
 12. A method of writing a memory array, comprising:creating a first voltage differential between a first column-supply lineand a second column-supply line; and, creating a second voltagedifferential between a first row-supply line and a second row-supplyline.
 13. The method of claim 12 wherein said first voltage differentialis at least a first threshold voltage of a first type of field-effecttransistor in magnitude and said second voltage differential is lessthan said a second threshold voltage of a second type of field-effecttransistor in magnitude.
 14. The method of claim 12 wherein said firstcolumn-supply line and said first row-supply line are connected to afirst inverter and said first voltage differential comprises said firstcolumn-supply line being at a greater voltage than said secondcolumn-supply line and said second voltage differential comprises saidfirst row-supply line being at a greater voltage than said secondrow-supply line.
 15. The method of claime 14, further comprising:creating a third voltage differential between said first column-supplyline and said second column-supply line; and, creating a fourth voltagedifferential between said first row-supply line and said secondrow-supply line, wherein said third voltage differential comprises saidsecond column-supply line being at a greater voltage than said-firstcolumn-supply line and said fourth voltage differential comprises saidsecond row-supply line being at a greater voltage than said firstrow-supply line.
 16. The method of claim 12, further comprising:creating a third voltage differential between said first column-supplyline and said second column-supply line; and, creating a fourth voltagedifferential between a first row-supply line and a second row-supplyline wherein said third voltage differential is opposite in polarityfrom said first voltage differential and said fourth voltagedifferential is opposite in polarity from said second voltagedifferential.
 17. The method of claim 16 wherein said first voltagedifferential is at least a first threshold voltage of a first type offield-effect transistor in magnitude and said second voltagedifferential is less than said a second threshold voltage of a secondtype of field-effect transistor in magnitude.
 18. A method of writing amemory array, comprising: driving a first voltage differential between afirst supply line and a second supply line wherein said first supplyline is connected to a first inverter in a plurality cells of a columnand said second supply line is connected to a second inverter in aplurality of cells of said column thereby causing a first current toflow between said first supply line and at least a third supply linewhen a cell of said plurality of cells in said column is in a firststate; and, driving a second voltage differential between said thirdsupply line and a fourth supply line thereby causing said cell to switchbetween said first state and a second state and wherein said thirdsupply line is connected to said first inverter-and said fourth supplyis connected to said second inverter.
 19. The method of claim 18,wherein said step of driving a first voltage differential comprisesincreasing a voltage on said first supply line relative to said secondsupply line.
 20. The method of claim 18, wherein said step of driving asecond voltage differential comprises decreasing a voltage on said thirdsupply line relative to said fourth supply line.
 21. A method of writinga memory array, comprising: supplying a first supply line and a secondsupply line with a first operating voltage; supplying a third supplyline and a fourth supply line with a second operating voltage; supplyinga first voltage differential between said first supply line and saidsecond supply line that is at least a threshold voltage of a first typeof field-effect transistor in magnitude thereby causing at least onecell to conduct a detectable amount of current between said first supplyline and said third supply line; and, supplying a second voltagedifferential between said third supply line and said fourth supply linethat is at least a threshold voltage of a second type of field-effecttransistor in magnitude thereby causing at least one cell to change froma first logic state to a second logic state.
 22. The method of claim 21wherein said first voltage differential comprises said first supply linebeing at a greater voltage than said second supply line.
 23. The methodof claim 22 wherein said second voltage differential comprises saidfourth supply line being at a greater voltage than said third supplyline.
 24. The method of claim 21 wherein said first voltage differentialcomprises said second supply line being at a greater voltage than saidfirst supply line.
 25. The method of claim 21 wherein said secondvoltage differential comprises said third supply line being at a greatervoltage than said fourth supply line.
 26. The method of claim 21 whereinsaid second voltage differential comprise said fourth supply line beingat a greater voltage than said third supply line.
 27. The method ofclaim 21 wherein said second voltage differential comprises said fourthsupply line being at a greater voltage than said third supply line.